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If you want an accurate Schmitt function, rather than a fast one...

what do you use?

I want to know the high and low switch thresholds, precisely, and for them not to drift.

A '555 relies on its power rails for thresholds, and an op amp needs rail/rail output for the positive feedback to be accurate, neither of which is ideal for a timing component. The old LM322 had a regulator, at least. It's as dead as the XR2206, though.

You can drive the '555 reference, but that's a tad messy.

So, what IS still available that's near ideal, in IC form? Three resistors, a TL431, a dual comparator and an R/S flop would be more messy.

Reply to
whit3rd
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If you are doing timing with an RC, you can charge the cap through a resistor from the same voltage that sets the thresholds. Then the power supply voltage drifts don't matter much.

We use an RC and a dac+comparator to make programmable time delays. If the charging supply is ratiometric on the DAC reference, it's stable.

Reply to
jlarkin

Available and near ideal (but requires an external reference and $$) LTC1042

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I've used an non-RR op-amp switching a CMOS analog switch, say 1/3 of a 4053. You can use a voltage divider from a reference for the thresholds. Easy-peasy.

Reply to
Spehro Pefhany

Cheep and cheerful:

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Reply to
bitrex

An RRO comparator such as a TLV7031 and two resistors. Run the op amp off an LP2951 to get decent rails.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

comparator

Reply to
Phil Hobbs

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